linux-kernel - Re: [RFC PATCH v2 2/3] mm: handle large folio when large folio in VM

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Date:   Fri, 14 Jul 2023 13:45:38 +0800
From:   "Yin, Fengwei" <fengwei.yin@...el.com>
To:     Hugh Dickins <hughd@...gle.com>
CC:     Yu Zhao <yuzhao@...gle.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <akpm@...ux-foundation.org>,
        <willy@...radead.org>, <david@...hat.com>, <ryan.roberts@....com>,
        <shy828301@...il.com>
Subject: Re: [RFC PATCH v2 2/3] mm: handle large folio when large folio in
 VM_LOCKED VMA range



On 7/14/2023 11:41 AM, Hugh Dickins wrote:
> On Fri, 14 Jul 2023, Yin, Fengwei wrote:
>> On 7/14/2023 10:21 AM, Hugh Dickins wrote:
>>> On Wed, 12 Jul 2023, Yin Fengwei wrote:
>>>> On 7/12/23 14:23, Yu Zhao wrote:
>>>>> On Wed, Jul 12, 2023 at 12:02 AM Yin Fengwei <fengwei.yin@...el.com> wrote:
>>>>>> --- a/
>>>>>> +++ b/mm/internal.h
>>>>>> @@ -643,7 +643,8 @@ static inline void mlock_vma_folio(struct folio *folio,
>>>>>>          *    still be set while VM_SPECIAL bits are added: so ignore it then.
>>>>>>          */
>>>>>>         if (unlikely((vma->vm_flags & (VM_LOCKED|VM_SPECIAL)) == VM_LOCKED) &&
>>>>>> -           (compound || !folio_test_large(folio)))
>>>>>> +           (compound || !folio_test_large(folio) ||
>>>>>> +           folio_in_range(folio, vma, vma->vm_start, vma->vm_end)))
>>>>>>                 mlock_folio(folio);
>>>>>>  }
>>>>>
>>>>> This can be simplified:
>>>>> 1. remove the compound parameter
>>>> Yes. There is not difference here for pmd mapping of THPs and pte mappings of THPs
>>>> if the only condition need check is whether the folio is within VMA range or not.
>>>>
>>>> But let me add Huge for confirmation.
>>>
>>> I'm not sure what it is that you need me to confirm: if the folio fits
>>> within the vma, then the folio fits within the vma, pmd-mapped or not.
>> Sorry. My bad. I should speak it out for what I want your confirmation:
>>   Whether we can remove the compound and use whether folio is within
>>   VMA instead.
>>
>> I suppose you answer is Yes.
> 
> Yes (if it all works out going that way).
> 
>>
>>>
>>> (And I agree with Yu that it's better to drop the folio_test_large()
>>> check too.)
>> My argument was folio_test_large() can filter the normal 4K page out so
>> it doesn't need to call folio_in_range() which looks to me a little bit
>> heavy for normal 4K page. And the deal was move folio_test_large()
>> to folio_in_range() like function so simplify the code in caller side.
> 
> I realized that, but agree with Yu.
OK. I will rethink this as both Yu and you suggested same thing.

> 
> It looked a little over-engineered to me, but I didn't spend long enough
> looking to understand why there's folio_within_vma() distinct from
> folio_in_range(), when everyone(?) calls folio_in_range() with the same
> arguments vma->vm_start, vma->vm_end.
madvise could call folio_in_range() with start/end from user space instead
of using VMA range.


Regards
Yin, Fengwei

> 
>>
>>>
>>> This idea, of counting the folio as mlocked according to whether the
>>> whole folio fits within the vma, does seem a good idea to me: worth
>>> pursuing.  But whether the implementation adds up and works out, I
>>> have not checked.  It was always difficult to arrive at a satisfactory
>>> compromise in mlocking compound pages: I hope this way does work out.
>> This is the purpose of this patch. :). Thanks.
>>
>>
>> Regards
>> Yin, Fengwei
>>
>>>
>>> Hugh